This invention relates to an input distortion correction current-feedback amplifier system.
Current feedback amplifier systems typically employ one or more signal current mirrors, an input stage coupled to the input of the current mirror, an output stage coupled to the output of the current mirror and a compensation impedance typically a capacitor which is charged by the current mirror to the voltage required to operate the output stage. A feedback current flows from the output stage to the inverting input of the input stage through a feedback element, typically a resistor. Ideally the system has a high impedance input for receiving signals with a low current drive capability and linearly reproduces those signals at its output with a high current drive capability. A problem arises however because transistor xcex94Vbe""s introduce distortion in the input stage resulting in non-linear performance.
It is therefore an object of this invention to provide an improved input distortion correction amplifier system.
It is a further object of this invention to provide such an improved input distortion correction amplifier system compatible with current feedback amplifiers.
It is a further object of this invention to provide such an improved input distortion correction amplifier system which is simple and effective and doesn""t limit voltage headroom.
It is a further object of this invention to provide such an improved input distortion correction amplifier system which corrects for the distortion at the compensating impedance and does not interfere with the input stage.
It is a further object of this invention to provide such an improved input distortion correction amplifier system which directly senses the distortion voltage at the input stage.
It is a further object of this invention to provide such an improved input distortion correction amplifier system which does not introduce common mode errors.
This invention results from the realization that input distortion correction can be achieved simply and effectively in current feedback amplifier systems by sensing the distortion voltage across the input stage, which induces a distortion current in the feedback circuit, developing a current representative of the distortion voltage and submitting that current to a current mirror which develops a correction current equal and opposite to the distortion current for charging the compensation capacitor to provide to the output stage a voltage input that nullifies the effect of the distortion voltage.
This invention features an input distortion correction current-feedback amplifier system including a current mirror, an input stage connected to the input of the current mirror, and output stage connected to the output of the current mirror. A feedback circuit is connected from the output stage to the input stage. A compensation impedance is connected to the output of the current mirror. A distortion correction circuit senses the distortion voltage across the input stage and provides to the current mirror a current representative of the distortion voltage for delivery to the compensation impedance for delivering to the compensation impedance a correction current to develop a correction voltage at the input of the output stage to nullify the effect of the distortion voltage.
In a preferred embodiment the distortion voltage may induce a distortion current in the feedback circuit and the correction current may be equal and opposite to that distortion current. The compensation impedance may include a capacitor. The current mirror may include a pair of current mirror circuits. The distortion correction circuit may include a pair of buffer amplifiers with their low impedance outputs connected to a correction resistance and at least one of their high impedance outputs connected to the current mirror. The pair of buffer amplifiers may have left to right symmetry for reducing common mode errors. The feedback circuit may include a feedback and a gain resistance and the correction resistance may have a value equal to twice the parallel combination of the feedback and gain resistances. The gain resistance may approach an open circuit. The distortion correction circuit may include a buffer amplifier with its low impedance output connected to a correction resistance and at least one of its high impedance outputs connected to the current mirror. The feedback circuit may include a feedback and a gain resistance and the correction resistance may have a value equal to the parallel combination of the feedback and gain resistance.